1. Field of the Invention
The present invention relates to a diffusion tube particularly useful in a diffusion furnace in which a semiconductor is heat-treated. More particularly, the present invention relates to a silicon carbide type diffusion tube that will not allow silicon wafers to have defects as a result of the presence of impurities.
2. Discussion of Related Art
Conventional types of diffusion tubes useful in a semiconductor diffusion furnace, include a quartz diffusion tube or a silicon carbide type diffusion tube. When these diffusion tubes are used to heat-treat a semiconductor, impurities present in the diffusion tube cause the silicon wafer to have defects, considerably lowering the yield of the semiconductor in the heat-treatment step. With respect to the purity, since quartz is superior to silicon carbide, a quartz diffusion tube is generally used.
However, even quartz is not satisfactory enough with respect to purity. Further, quartz diffusion tubes are liable to deform at a high temperature. Therefore, a defect of such quartz diffusion tubes is that the useful life thereof is short. Particularly, when the treating temperature is over 1,200.degree. C., the wear is severe due to the deformation, devitrification, etc., so that quartz diffusion tubes have to be replaced frequently. On the other hand, if the treating temperature is lowered in order to avoid the problems of deformation and devitrification of the quartz diffusion tubes, the treating time must be extended considerably, resulting in a high cost of production of the semiconductor.
In contrast, a silicon carbide type diffusion tube hardly deform even at a high temperature, and devitrification observed within a quartz diffusion tube will not take place in the case of a silicon carbide type diffusion tube. The usable period of a silicon carbide type diffusion tube can be extended considerably in comparison with a quartz diffusion tube. However, with respect to the purity, since a silicon carbide type diffusion tube in the prior art is inferior to a quartz diffusion tube, the application of a silicon carbide type diffusion tube is limited.